Differential transformer with balancing means

ABSTRACT

A differential transformer having a balancing sleeve interposed between its primary and secondary windings to provide a substantially identical degree of coupling between each of the primary windings and the secondary winding. The balancing sleeve is made of magnetic and/or electrically conductive materials.

United States Patent Douglas et al.

115 1 3,665,356 1451 May 23, 1972 [54] DIFFERENTIAL TRANSFORMER WIBALANCING MEANS I [72] inventors: Ellwood S. Douglas, Orinda; Wallace W.

Wahlgren, Oakland, both of Calif. [73] Assignee: The Rucker Company,Oakland, Calif.

[22] Filed: Apr. 23, 1969 [2 l] Appl. No.: 818,558

52 us. c1 ..336/73, 33 /84, 336/175,

7 336/212 511 1111.0. ..H01fl7/06 [58] FieldofSearch..336/173,l74,l75,84,2l2, 336/73 [56] References Cited I UNITED STATESPATENTS 3,525,964 8/1970 Stevenson ..336/84 Brennan et a1 ..336/73433,702 8/1890 Tesla ..336l212 X 1,832,662 11/1931 Schmutz.. ..336/843,296,364 1/1967 Mason ..336/174 X 3,343,074 9/ l 967 Brock 336/175 XFOREIGN PATENTS OR APPLICATIONS 288,606 3/1929 Great Britain .i ..336/1.75

1,086,341 8/1960 Germany ..336/ l 74 Primary Examiner-Thomas J. KozmaAttorney-Elem, l-lohbach, Test, Albritton 8L Herbert 57 7 ABSTRACT Adifferential transformer having a balancing sleeve interposed betweenits primary and secondary windings to provide a substantially identicaldegree of coupling between each of theprimary windings and the secondarywinding. The balancing sleeve is made of magnetic and/or electricallyconductive materials.

2 Claims, Drawing Figures Patented May 23, 1972 3,665,356

2 Sheets-Sheet l INVENTORS ELLWOOD S. DOUGLAS BY WALLACE W WAH LGRENATTORNEYS Patented May 23, 1972 2 Sheets-Sheet 2 INVENTORS ELLWOOD S.DOUGLAS BY WALLACE W WAHLGREN ATTORNEYS 1: DIFFERENTIAL TRANSFORMER wrrnBALANCING MEANS v BACKGROUND OF THE INVENTION l-leretofore, differentialtransformers have been provided wherein a currentis induced in asecondary winding in proportion to the difference between the currentsflowing in two or more primary windings. A problem exists in suchtransformers in providing balanced coupling between each of the primarywindings and the secondary winding' This problem is particularlysevere'with differential trans-- formers whichare used in ground faultdetectors of the type described in US. Pat. No. 3,213,321, issued Oct.19, 1965. Such transformers must be capable of detecting leakagecurrents on the order of 5 milliamperes. Thus, for typical primarycurrents on the order of 100 amperes, a sensitivity on the order of 50parts in a million is required. In order to have a sensitivity ratio ofthis magnitude, it is necessary to'have a very high degree of balanceamong the primary windings of the transformer. The magnitude of therequisite degree of balance becomes apparent when one considers that thecurrent due to unbalanced coupling should be at least an order ofmagnitude smaller than the leakage current which is to be detected. In

the example given, the ratio of current due to unbalanced SUMMARY ANDOBJECTS OF THE INVENTION The present invention provides a differentialtransformer having a plurality'of single turn primary windings, at leastone secondary winding, and balancing means interposed between theprimary and secondary windings to provide a substantially identicaldegree of coupling between each of the primary windings and thesecondary winding.

It is in general an object of the'present invention to provide adifferential transformer wherein a substantially identical degree ofcoupling is provided between each of the primary windings and thesecondary winding.

Another object of the invention is to provide a differentialtransformerof the above character which can be readily and economicallyproduced.

Additional objects and features of the invention will be apparent fromthe following specification in which the preferred embodiments of theinvention are described in detail and illustrated in the accompanyingdrawing.

BRIEF DESCRIPTION OF THE DRAWING ferential transformer incorporating thepresent invention.

FIG. 5 is a sectional view of a fourth embodiment of a differentialtransformer incorporating the present invention.

FIG. 6 is a sectional view of a fifth embodiment of a differentialtransformer incorporating the present invention.

Like reference numerals are used to designate corresponding elements ineach figure of the drawing.

DESCRXPT ION OF THE PREFERRED EMBODIMENTS The embodiment of thedifi'erential transformer shown in FIGS. 1 and 2 comprises generally atoroidal core 10, primary windings 11, a secondary winding 12, and abalancing sleeve 13.

Toroidal core 10 is made of a material having a high magneticpermeability, such' as supermalloy. Preferably, this core should be asnearly circular in shape as possible and should have a uniform crosssection. While these core dimensions are not as critical as they wouldbe in a conventional differential transformer, some improvement inbalance may nevertheless be obtained by proper attention to the symmetryof the unit.

Primary windings 11 consist of a plurality of current-carryingconductors which pass through the opening in toroidal core 10, therebyforming a plurality of single turn primary windings. The size of theseconductors is dependent upon the currents which they must carry, and thenumber of windings'is determined by the application for which thetransformer is intended. The embodiment shown has four primary windingsand, hence, is suitable for a three-phase, four-wire system.

While the placementof the wires is not as critical as it would be in aconventional differential transformer, some improvement in balance maybe realized by arranging primary windings 11 in a symmetrical manner.ideally, all primary windings should lie exactly on the axis of toroidalcore 10. In a three-phase, four-wire system, the neutral or returnconductor can be located at or near' the axis, with the remainingconductors being distributed around it in a symmetrical pattern. Someadditional improvement in balance can be provided bytwisting theconductors together in the vicinity of toroidal core 10.

The secondary winding 12 consists of a plurality of turns wound on thetoroidal core 10. A pair of leads 14 is provided for connection to thesecondary winding. Preferably, 'the secondary turns should be tightlywound and evenly distributed about the toroidal core. In addition, it isdesirable that the secondary winding occupy an integral number of layerson the core. in transformers having more than one secondary winding, itis desirable. that each of these windings be tightly and symmetricallywound.

The balancing sleeve 13 is disposed between the primary windings 11 andthe secondary winding 12. Sleeve 13 is generally cylindrical in shape.It encircles primary windings 1 l in the vicinity of toroidal core 10and passes through the opening in the core. Preferably, the axis ofsleeve 13 should coincide with the axis of toroidal core 10.

The dimensions of balancing sleeve 13 are not critical. It has beenobserved, however, that more balanced coupling is obtained when sleeve13 has an external diameter approaching the internal diameter of core10. Likewise, the degree of balance increases as the wall thickness ofsleeve '13 is increased. Satisfactory results have been obtained with awall thickness as small as 0.050 of an inch.

The improvement in balance provided by sleeve 13 increases withincertain limits as sleeve 13 is increased in length. It has been observedthat the improvement in balance decreases sharply as tube 13 is madeshorter than the width of toroidal core 10. Thus, in the preferredembodiment, sleeve l3 should-be at least as long as core 10 is wide. lthas also been observed that little additional improvement in balance isprovided by making sleeve 13 more than about three times as long as core10 is wide. The axial placement of sleeve 13 is not critical as long asthe sleeve extends completely through core 10. In the preferredembodiment, the sleeve is centered within the core.

The balancing sleeve 13 can be fabricated of any material having a highmagnetic permeability. This magnetic material apparently serves to shortout any uncancelled fluxes from the primary windings before they canreach the secondary winding. Although this magnetic sleeve eliminates asubstantial portion of the uncalcelled fluxes from the primary windings,it has been observed to have no significant effect upon the sensitivityof the transformer to the leakage currents to be detected. While theremay be a slight reduction in the sensitivity of the transformer, thisreduction is almost undetectible in view of the ratio of the crosssection of the balancing sleeve to the cross section of the toroidalcore.

An additional improvement in the coupling balance can be provided byfabricating balancing sleeve 13 of a material which is electricallyconductive, as well as having a high magneticpermeability, Suitablematerials having these two properties include soft iron, nickel, andnickel-iron alloys, such as permalloy and supermalloy. It is thoughtthat the electrical conductor provides a path for eddy currents, so thatthe eddy currents can distribute themselvesin such a manner as toimprove the coupling balance.

The various elements of the differential transformer can be assembledand held together by conventional means. One particularly effective andconvenient means is to.pot" the entir assembly in an epoxy rosin.

FIG. 3 shows another embodiment of the present invention which issimilar to that previously described except for the construction of thebalancing means 13. In this embodiment, balancing means 13 comprises anannular cylindrical sleeve 16 and an outer cylindrical sleeve 17.Sleeves l6 and 17 are concentric with respect to each other and withrespect to the axis of toroidal core 10. Sleeve 16 is fabricated of anelectrically conductive material, such as copper, and sleeve 17 isfabricated of a material having a high magnetic permeability.

.This embodiment has been found to provide a greater improvement in thebalance of the coupling between the primary and the secondary windingsthan is provided by the embodiment shown in FIG. 1.

In the embodiment shown in FIG. 4, the balancing means 13 includes aninner electrically conductive sleeve 21 and an outer electricallyconductive sleeve 22. These sleeves are generally cylindrical in shapeand are concentric with respect to each other and with respect to theaxis of toroidal core 10. Sleeves 21 and 22 are joined together at theirends by annular rings 23. Rings 23 are also made of an electricallyconductive material and are joined to sleeves 21 and 22 by conventionalmeans, such as soldering. A sleeve of magnetic material 24 is disposedin the space defined between sleeves 21 and 22 and rings 23. Theeffectiveness of this embodiment in improving the balance in couplingbetween the primary and secondary windings has been found to becomparable to that of the em bodiment shown in FIG. 3.

Operation of the embodiment shown in FIG. 4 is believed to be asfollows. Current flowing in the primary conductors ll induces a flux inthe magnetic sleeve 24. .The rate of change of this flux induces avoltage from end-to-end in the inner conductive sleeve 21. This voltageforces current to flow from end-to-end in the outer conductive sleeve22, returning through the inner sleeve. The induced current in the innersleeve 2] is in a direction opposite to that of the current in theprimary windings. Where the sleeve assembly is provided with adequatecore material and conductive sleeves, the induced current will be nearlyas large as the primary current, thereby reducing the flux in themagnetic sleeve 24 to a value much less than it would be with noconductive sleeves andno in duced currents. Thus, the magnetomotiveforce seen by toroidal transformer core 10 is mostly due to the inducedcurrent flowing in the outer conductive sleeve 22. If one of primarywindings 11 is in a position such that its coupling to inner conductivesleeve 21 varies around the sleeve, the induced current will havecircumferential components at the ends of the sleeves which distributethe current more evenly over the outer sleeve 22. Thus, the net resultof using this sleeve assembly is an improvement in transformer balancewithout any substantial effect on the sensitivity of the transformer tothe leakage currents to be detected.

The embodiment shown in FIG. 5 is similar to those previously describedexcept for the construction of the balancing sleeve 13. The balancingsleeve is formed to include a substantially cylindrical central portion26 and flared end portions 27. This embodiment can be constructed 'morecompactly than those previously described'The central sleeve portion 26can have a length corresponding generally to the width of the toroidalcore 10,- and the flared portions 27 should preferably extend radiallyoutward to the outer surface of the toroidal core. The flared sleeve ismost convenientlylfabricated in two sections which can be inserted intothe toroidal core 10 from by the flared sleeve is comparable to thatprovided by the other embodiments heretofore described.

The embodiment shown in FIG. 6 is very similar to that shown in FIG. 5.In FIG. 6, however, the balancing sleeve 13 includes a centralcylindrical member 31 and annular end rings 32. The end rings 32 arefitted over the end portions of the central member 31 and joined theretoby conventional means. The sleeve 13 in this embodiment can befabricated of the same types of materials as the sleeve in the FIG. 5embodiment, with a comparable improvement in the coupling balance. I

While the preferred embodiments have been described in terms ofdifierential transformers having toroidal cores, it is to be understoodthat the present invention is not limited to this particulartype oftransformer. Thus, for example, the invention can also be utilized indifferential transformers having a rectangular core, in which case thebalancing sleeve would preferably be rectangularin cross section tocorrespond to the shape of the core.

It is apparent from the foregoing that there has been provided a new andimproved differential transformer with balancing means which provides asubstantially identical degree of coupling between each of thetransformers, primary a plurality of conductors passing through saidcore to form a plurality of single turn primary windings, a secondarywinding wound on said core, and current balancing means surrounding saidconductors and extending through said core to provide a substantiallyidentical degree of coupling between each of said primary windings andsaid secondary winding, said current balancing means including spacedapart inner and outer sleeve portions fabricated of an electricallyconductive material disposed coaxially of said core, means electricallyconnecting said inner and outer sleeve portions together at their ends,and a sleeve of magnetic material intermediate said inner and outersleeve portions, said sleeve extending 'coaxially of said core.

2. A differential transformer as in claim 1 wherein said inner and outersleeve portions are cylindrical sleeves and the means connecting theends of said sleeve portions together includes annular rings extendingbetween said sleeves at the ta us w

1. In a differential transformer, a core of magnetic material, a plurality of conductors passing through said core to form a plurality of single turn primary windings, a secondary winding wound on said core, and current balancing means surrounding said conductors and extending through said core to provide a substantially identical degree of coupling between each of said primary windings and said secondary winding, said current balancing means including spaced apart inner and outer sleeve portions fabricated of an electrically conductive material disposed coaxially of said core, means electrically connecting said inner and outer sleeve portions together at their ends, and a sleeve of magnetic material intermediate said inner and outer sleeve portions, said sleeve extending coaxially of said core.
 2. A differential transformer as in claim 1 wherein said inner and outer sleeve portions are cylindrical sleeveS and the means connecting the ends of said sleeve portions together includes annular rings extending between said sleeves at the ends thereof. 